Abstract: We report on a joint theoretical and experimental study of an integrated
photonic device consisting of a single mode waveguide vertically coupled to a
disk-shaped microresonator. Starting from the general theory of open systems,
we show how the presence of a neighboring waveguide induces reactive inter-mode
coupling in the resonator, analogous to an off-diagonal Lamb shift from atomic
physics. Observable consequences of this coupling manifest as peculiar Fano
lineshapes in the waveguide transmission spectra. The theoretical predictions
are validated by full vectorial 3D finite element numerical simulations and are
confirmed by the experiments.